Time Space Time (TST) switches are a particularly useful configuration of switching elements providing both time and space translation between channels of Time Division Multiplexed (TDM) telecommunications transmission lines using lightwave transmission facilities. A TST switch interconnects digital bi-directional TDM communication lines with TDM communication involving the sharing of single transmission paths, individually, in time to provide multiple channels in a single transmission medium. This is a fundamental system improvement in telephone communications to reduce cost of ordinary telephone service, and in enhancing the ability to provide many new kinds of services, in meeting expanded communications needs.
Electromechanical crossbar and relay switching systems, as generally used today in telecommunications switching, have, for practical purposes, reached the limit of their capabilities. Extensive, continued adherence to these older technologies severely restricts capability, and greatly increases costs of telecommunication systems; and, particularly so with expansion to systems of great size and complexity. While many advances have been made in capability and efficiency in the transmission area, with microwave, satellite, and high-capacity cable, and with both analog and digital repeaters and terminals being used, the exchange plant, including switching equipment in central offices and branch exchanges, remain in essence the same as in the very early days. Recent advances in solid state technology make the use of all digital switching and transmission techniques more attractive today than ever before.
The advent of digital multiplex transmission systems gives rise to many possibilities; particularly with TDM multiplex terminals beginning to look like switches. Message signals in these terminals appear in "time slots," and transfer of signals between time slots is accomplished by a "time slot interchange," with time-division switches connected directly to multiplex transmission lines. Another important saving is accomplished through elimination of digital-to-analog, and analog-to-digital, conversions of every switch. The existing local exchange area plant represents the major part of telephone plant investment, and the least efficient portion of the system--with large quantities of scarce materials required. Further, physical congestion problems are encountered with entrance cables as they approach the central office, and, many times, there are difficult growth problems in central office main distribution frames.
Present telephone central office switching includes bulky electromechanical switching stages located in large, costly building space. Costs for new construction and maintenance of such traditional exchange area plants are constantly increasing, particularly with large cable networks employed when cable pair utilization is inherently very low with a dedicated physical wire pair used to connect each subscriber station to its central office. Thus, system improvements attainable with time division transmission and switching techniques are very significant, and have resulted in the development of TST switches and systems described and claimed in, for example U.S. Pat. Nos. 3,925,621; 3,956,593; 4,005,272; and 4,038,497.
The development of fiber optic lightwave transmission of information is a truly qualitative advance. The bandwidth limited channel capacity inherent in metallic media transmission has been removed. A single monomode optical fiber can carry many thousand distinct digital voice and data channels per fiber. Once a fiber optic cable is in place, increases in channel capacity are essentially cost free. At present, there is a belated effort to install digital switches in an outmoded 1877 structural concept and obsolete 64-kb/s T1-D4 copper wire based carrier channels.
The telephone industry has been slow to adopt advanced technology concepts that would enhance the capacity, speed and usefulness of telephone networks to users. TST switches are coming into wider use in the telephone network, but full benefits of such switches have not been realized. In recent years fiber optic cable and equipment have been developed for commercial application. Fiber optic communications can make possible extremely high data transmission rates, greater than communication rates that can be achieved with present metallic conductors.
The present architecture of the telephone system does not fully exploit either the capabilities of digital TST switches or the enormous capacity of fiber optic communications. These two developments have dramatically changed the economics of the telephone network. However, development of new kinds of switches that will be required to selectively connect telephones, terminal stations and computers on demand thus making lightwave transmission facilities useful as a switched network has been lacking. Much more than one-for-one replacement of copper wire with glass fiber is required.
As a result of these developments there is a need for a telephone network architecture which can take advantage of the lower cost TST switches and the extremely high data transmission rates availability with fiber optic technology as well as improved TST switches.
The present invention is also of particular significance in view of the increasing use of digital transmission and switching in voice and data telecommunication networks and the need for accurate information transmission through these networks. Heretofore, digital transmission systems have relied on "pulse stuffing" techniques to ensure accurate transmission of information in a digital multiplex hierarchy.
Various approaches to network synchronization have been proposed to equalize clocks throughout a large network. One such approach is shown in U.S. Pat. No. 4,270,203. Master/slave arrangements are possible wherein, for example, a relatively expensive atomic standard is used as a clock source and other clocks are slaved to this master. Mutually synchronized clock systems have also been studied with individual clocks arranged in a manner that attempts to achieve an average network frequency based on information from each clock. The use of highly stable, independent clocks is yet another possibility. Even with these measures, it is not possible to completely control clock signals and synchronization errors will eventually occur. A need has thus arisen for an improved synchronization arrangement.